What is the physical difference between the Subsurface Scattering and Translucent shaders?
Translucency allows light to pass through an object. Using only a translucent shader will leave an object dark on the side receiving light with the other sides being lit.



Sub Surface Scattering allows light to penetrate the surface and be distorted before bouncing back out. Each colour component disperses differently, leaving areas with a different colour than others. Thin areas allow light to pass through to light the opposite side before being bounced back out, these thin areas give a translucent appearance.